Unique flake-shaped sulfur morphology favored by the binder-free electrophoretically deposited TiO2 layer as a promising cathode structure for Li-S batteries

Abstract

Metal oxides have been extensively studied as catalyst additives in Li-S batteries due to their high affinity for soluble lithium polysulfides. The present study developed a novel carbon nanomaterials- and binder-free 3D sulfur host by electrophoretic deposition (EPD) of TiO2 nanoparticles on carbon fiber paper (CFP). Benefiting from the unique well-connected TiO2 porous layer formed by EPD, the initial discharge capacity of Li-S battery was increased from 1160 mAh g-1 in conventional casted TiO2/binder/S cathode (Cast:CFP/PVDF/TiO2/S) to 1310 mAh g-1 in the EPD:CFP/TiO2/S cathode, along with an overpotential drop of 16%. Unlike typical sulfur particulates found in the Cast:CFP/PVDF/TiO2/S samples, flake-shaped sulfur precipitates were observed in the binder-free EPD:CFP/TiO2/S electrode at the end of charge. Pulverization of sulfur was effectively suppressed in the EPD:CFP/TiO2/S electrode owing to the formation of the flake-shaped sulfur morphology, leading to superior electrochemical performance and better cycle life. Microstructural studies and real-time EIS investigations revealed that sulfur flakes nucleated across the Li2S matrix at the initial stages of charge and grew vertically from the sulfur/TiO2 interface. The preferential growth mechanism of sulfur flakes was attributed to the surface conductance of the EPD-TiO2 film along the ordered clusters formed during EPD, enhancing the kinetics of electrocatalytic reactions of polysulfides at the TiO2/sulfur interface. Our results show that developing novel methods to modify the morphology of secondary sulfur upon cycling is critical for enhancing charge-discharge performance in Li-S batteries.